A discriminator, in some suitable form, is a key element of an instantaneous frequency measuring (IFM) receiver. A receiver of the IFM type has the ability to monitor continuously all frequencies within a designated frequency band, a property very useful for certain types of surveillance operations. The IFM receiver's military use has become widespread at microwave frequencies up to 18GHz. Extension of this coverage upward in frequency has been hindered to a large extent by technical limitations of available components.
A discriminator of the type used for IFM receiver applications is referred to herein as an instantaneous frequency discriminator (IFD), the function of which is to detect the presence of a signal and to output the frequency and power characteristics of that signal. The IFD is distinguished from other types of discriminators such as, for example, those used for FM signal demodulation which are used to provide an output proportional to the frequency of a demodulated signal.
The IFD responds to an input RF signal so as to communicate the frequency and power level in the form of a visual polar display. In such a display, the angle of a radial strobe has a one-to-one correspondence with a given input signal's frequency and the length of the strobe's radius vector is proportional to a given input signal's power level. Use of a single IFD to cover a very wide band of frequencies is often desirable from a practical standpoint and a capability of operating over a 3:2 band of frequencies is considered a reasonable span of coverage.
Most previous IFD's have been confined to the conventional microwave bands, i.e., mostly to frequencies below 18GHz and require the use of a separate, frequency independent 90.degree. phase shifter. Attempts to attain higher frequency operation have in the past been plagued by a variety of problems. Some of the most serious of these problems stem from generally poor electrical characteristics of components and interconnecting circuitry at the frequencies of interest, together with fabrication difficulties arising from physically small circuits with very close tolerances. Conventional strip line techniques tend to become impractical at extremely high frequencies. Devices such as, for example, coupled-strip line phase shifters and hybrids, widely used at frequencies below 18GHz, are very difficult to fabricate because of dimensional constraints and extremely close proximity of conductors and are therefore unsuitable for discriminators which must operate at frequencies above 18GHz.